[pursuant to NOT-AG-18-008 submission instructions, this Supplement Abstract reflects the relevance of the proposed research to AD/ADRD and so is updated from the parent-grant Abstract. However, the scope of the parent-grant is unchanged as funded. In the Supplement Abstract, additions from the parent grant abstract in RED] Abstract The BRAIN initiative aiming to revolutionize understanding of the brain requires ?improvement of existing non-invasive neuromodulation? (RFA-MH-16-810). Arguably no existing technique in humans has generated more interest than transcranial Direct Current Stimulation (tDCS). tDCS applications span Alzheimer's Disease and its related Dementias (AD/ADRD). For the value of ongoing and future to be maximized, computational models must be adapted to tDCS in older individuals. Computational models of tDCS predict brain current-flow in individual subjects, and support the development of targeted montages. In older adults with brain atrophies highly conducting cerebrospinal fluid fills the void and significantly alters current flow in the brain. Through three innovations, this proposal removes barriers limiting access to computational models by tDCS researcher for this population. First, a decade of technical innovation in automated image segmentation and high-throughput current-flow modeling will be enhanced and encoded in cloud- enabled open-source. Through this supplement, we will further enhance reliability in modeling older adults. Second, state-of-the-art MRI mapping of tDCS current distribution will validate and refine model methods. Through this supplement, we will empirically validate altered current flow in adults with brain atrophy. Third, stand-alone and web-based modeling software will be deployed, with computationally demanding steps implemented on servers. Ongoing and future work testing tDCS for AD/ADRD will directly benefit. Directly responsive to the parent RFA, the outcome of this proposal is a toolbox for the optimization of tDCS spatial precision to enhance the rigor of tDCS research aimed at understanding the brain and treating disease. Directly responsive to the supplement RFA, the enhancements described here will stimulate additional activity leading to progress in AD/ADRD treatments using tDCS. While the supplement focuses on AD/ADRD, the work proposed is within the scope of the active award.